1. Field of the Invention
The present invention relates to a differential absorption lidar apparatus for transmitting laser light with at least two wavelengths in the air and receiving scattered light from a target (gas to be measured, such as carbon dioxide, ozone, or water vapor), and measuring the concentration of the target in the air from a difference in intensity of received light regarding each wavelength.
2. Related Background Art
A conventional differential absorption lidar apparatus transmits laser light with two wavelengths having very high coherency in the air, receives scattered light from the ground or an aerosol to be a target, and heterodyne-detects the received light and local light. The differential absorption lidar apparatus obtains a frequency spectrum of a signal obtained by heterodyne-detection, and detects received light regarding each of the two wavelengths from the frequency spectrum (see, for example, “Coherent differential absorption lidar measurements of CO2” (edited by G. J. Koch et al., Applied Optics, Vol. 43, No. 26, 10 Sep. 2004, pp. 5092-5099) and “Coherent Laser Radar Transceiver for the NASA CO2 Laser Absorption Spectrometer Instrument” (edited by M. W. Phillips et al., Proceedings of 13th Coherent Laser Radar Conference, 2005, pp. 118-121)).
At this time, among two wavelengths, one wavelength is set to be the one having a large absorption coefficient regarding gas to be measured, and the other wavelength is set to be the one having a small absorption coefficient. Owing to this setting, the amount of received light varies depending upon the concentration of gas to be measured between the apparatus and the target, and the concentration of the gas (specifically, the concentration of carbon gas) is measured from the difference.
The above-mentioned conventional differential absorption lidar apparatus is one type of a so-called coherent lidar. This type of lidar adopts a heterodyne-detection system that is likely to realize an ideal light reception state (a shot noise limit), and has an advantage in that a high S/N ratio is likely to be obtained in view of the reception, compared with an incoherent lidar known as the other type.
However, particularly in the case where the above-mentioned conventional differential absorption lidar apparatus is required to operate under very strict constraints (e.g., the apparatus is mounted on an artificial satellite to measure the concentration of gas in the vicinity of the ground), there arise the following problems.
First, in the coherent lidar, coherency at an optical carrier level of laser light to be transmitted/received is important, and it is also important to transmit/receive laser light having a very small line width. Furthermore, in order to perform a long-distance measurement of the order of 100 km as in the observation of the vicinity of the ground from the artificial satellite, a high transmission power of laser light is required. It is very difficult to allow laser light with such a small line width and a high power to be operated under strict constraints required when the coherent lidar is mounted on the satellite, which makes it difficult to maintain the reliability of the apparatus.
Furthermore, when a measurement is performed with the coherent lidar mounted on a moving body such as an artificial satellite, a Doppler frequency shift occurs in a light carrier due to the movement speed of the moving body itself. The Doppler frequency shift is 1.3 MHz with respect to the moving speed of 1 m/s, for example, in the case of an optical wavelength of 1.5 μm. That is, in view of the moving speed of the artificial satellite, a Doppler frequency shift of the order of 100 MHz may occur. However, it is difficult to successively grasp the Doppler frequency shift caused by the above-mentioned moving speed. Therefore, it is necessary to detect a component receiving an unknown frequency shift from a signal obtained by heterodyne-detection in a wide frequency range, which makes it difficult to detect a signal.
As described above, as a differential absorption lidar apparatus mounted on an artificial satellite required for satisfying strict constraints, the incoherent lidar is advantageous, which less requires a line width of laser light to be transmitted/received, and which is less influenced by a Doppler frequency shift.
However, assuming that the incoherent lidar adopts a pulse system (for example, transmits/receives a pulse corresponding to a distance resolution of about 150 m), a required reception bandwidth is about 1 MHz, and hence, a large reception bandwidth is required. According to a direct detection system used in light reception of the incoherent lidar, a reception S/N ratio is inversely proportional to the reception bandwidth. Thus, when the large reception bandwidth as described above is required, it is difficult to obtain a high reception S/N ratio.
Furthermore, if the incoherent lidar (see, for example, Technical Report 38 (edited by Murota et al., pp. 258-261, 2001 of Mitsubishi Heavy Industries Ltd.)) adopts a continuous wave (CW) system, a large bandwidth for responding to the above pulse is not required, so a reception bandwidth of a light receiver can be made small, and reception with high sensitivity becomes possible while direct detection is used in the light reception. However, this conventional differential absorption lidar apparatus has a problem regarding the function of receiving two wavelengths required for the apparatus. That is, the conventional differential absorption lidar apparatus has no function of identifying two wavelengths in reception, in the case where the incoherent lidar and the CW system are adopted, and two wavelengths are transmitted concurrently. Thus, if the CW system is used, there is no choice but to divide a timing for transmitting/receiving the two wavelengths in terms of the time, and transmit/receive them individually. According to such a configuration, in the case where the incoherent lidar mounted on an artificial satellite performs a measurement while moving, there arises the following problem.
In the case of a measurement while moving, according to the configuration in which two wavelengths are divided in terms of the time and transmitted/received individually, the irradiation position with respect to a target varies between two wavelength components. Generally, it is considered that the reflectance of a target varies depending upon the position. The differential absorption lidar apparatus measures the concentration of gas from the difference in the amount of received light between two wavelengths. Therefore, in order to perform a measurement with high precision, the dependence on only the difference in the amount of absorption in the air between two wavelengths is required as a precondition. When the reflectance varies in a measurement of the amount of received light at each wavelength, the precondition does not hold, which may degrade measurement precision.
For the above reasons, conventionally, in the measurement by the differential absorption lidar apparatus mounted on the artificial satellite required to satisfy strict constraints, the concentration of gas to be measured cannot be measured at a high reception S/N ratio and with high precision.